The present invention relates to PID-type controllers, and more particularly, to an automatically tuned PID-type controller and an adaptive control method therefore.
In a conventional feedback process control system, a proportional-plus-integral-plus-derivative (PID) operation is performed with respect to a deviation (e) between a set point (sp) and a controlled variable (pv) fed back from the process, and the result of the PID operation is supplied as a control signal to the process. In order to perform an optimum control of the process, it is necessary that the PID parameters for performing the respective PID operation are set to their optimum values. Conventionally, the PID parameters have been manually adjusted. For implementing the manual adjustment, a step response method and a marginal sensitivity method have been well-known. In both the methods, however, it takes a long time for the measurement of characteristic, and the process control is stopped while the measurement is effected so that the value of pv obtained at that time cannot be the most desirable one.
On the other hand, methods have been proposed in which the PID parameters are automatically determined (i.e., not manually determined.) Such an automatic method is described in U.S. Pat. No. 4,754,391. However, should any changes occur in the process, the controller must be retuned, the retuning being initiated manually.
In the adaptive controller and method of the present invention, the process output is monitored. Thus, if a process change takes place after line-out (i.e., after sp=pv) sufficient to cause a predetermined error without a corresponding change in the setpoint (sp), the adaptive controller of the present invention initiates an analysis of the process and retunes the PID parameters to optimize control of the process.